Process for treating a furan-2,5-dicarboxylic acid composition

ABSTRACT

A process for treating a furan-2,5-dicarboxylic acid composition, which process comprises: introducing a furan-2,5-di-carboxylic acid composition, which furan-2,5-dicarboxylic acid composition contains an impurity compound and which impurity compound is 5-formyl-furan-2-carboxylic acid, into a cathode compartment of an electrochemical cell; and electrochemically reducing the impurity compound in the cathode compartment.

FIELD OF THE INVENTION

The invention relates to a process for treating a furan-2,5-dicarboxylicacid composition.

BACKGROUND TO THE INVENTION

Dicarboxylic acids are widely used in the chemical industry as astarting material for the production of polymers.

An example is furan-2,5-dicarboxylic acid (FDCA) which can bepolymerized in the presence of ethylene glycol (EG) to form polyethylene furanoate (PEF).

The chemical preparation of polymer grade dicarboxylic acids, such asfor example polymer grade furan-2,5-dicarboxylic acid, from relativelyless pure or “crude” dicarboxylic acid is cumbersome.

In US 2013/0345452 it is explained that FDCA can be synthesized by thecatalytic oxidation of 5-(hydroxymethyl)furfural (HMF); or by thecatalytic oxidation of HMF esters (5-R(CO)OCH₂-furfural where R=alkyl,cycloalkyl and aryl); or by the catalytic oxidation of HMF ethers(5-R′OCH₂-furfural, where R′=alkyl, cycloalkyl and aryl); or by thecatalytic oxidation of 5-alkyl furfurals (5-R″-furfural, where R″=alkyl,cycloalkyl and aryl) whilst using a Co/Mn/Br catalyst system. Mixedfeedstocks of HMF and HMF esters, mixed feedstocks of HMF and HMFethers, and mixed feedstocks of HMF and 5-alkyl furfurals can also beused. US 2013/0345452 further describes a process for purifying a crudeFDCA composition by hydrogenation in the presence of a hydrogenationcatalyst in a hydrogenation solvent such as water at a temperaturewithin a range of 130° C. to 225° C. The hydrogenation needs to beconducted at a temperature where FDCA is sufficiently soluble in water.Unfortunately in the process of US 2013/0345452 not only2-formyl-furan-5-carboxylic acid is hydrogenated, but also2,5-furandicarboxylic acid appears to be partly hydrogenated intotetrahydrofuran-2,5-dicarboxylic Acid (THFDCA)

WO 2015/030590 explains that the purification of 2,5-furandicarboxylicacid containing compositions that also contain2-formyl-furan-5-carboxylic acid are difficult to purify byrecrystallization. WO 2015/030590 therefore proposes a process forpurifying an acid composition comprising 2-formyl-furan-5-carboxylicacid and 2,5-furandicarboxylic acid, which process comprises anesterification to obtain an esterified composition and subsequentseparation of the ester from the esterified composition. It is, however,evident that this proposed method remains cumbersome.

It would be an advancement in the art to provide a process for thetreatment of a furan-2,5-dicarboxylic acid composition which is lesscumbersome.

Non-pre published patent application WO2016/186504 describes a processwherein a feedstock comprising at least an aromatic aldehyde compound isintroduced into an electrolytic cell comprising electrodes, wherein atleast one of the electrodes comprises a non-noble metal and/or an oxideand/or a hydroxide thereof and/or carbon; and wherein the aromaticaldehyde compound is electrochemically oxidized to yield an aromaticdicarboxylic acid. According to examples 2 and 3 of WO2016/186504, theresulting products contained less color bodies. Non-pre published patentapplication WO2016/186505 describes a process for the purification of acarboxylic acid-containing composition, which composition furthercontains an aldehyde, which process comprises electrochemicallyoxidizing the aldehyde in an electrolytic cell to obtain anelectrochemically oxidized product composition comprising a carboxylicacid derived from the aldehyde and optionally separating carboxylic acidfrom the electrochemically oxidized product composition.

Although good results are obtained with the processes of WO2016/186504and WO2016/186505, there is still room for alternative processes andfurther improvement.

It would therefore be an advancement in the art to have a process withwhich similar, better, or economically more advantageous results can beachieved.

SUMMARY OF THE INVENTION

Such a process has been achieved with the process according to theinvention. Accordingly the present invention provides a process fortreating a furan-2,5-dicarboxylic acid composition, which processcomprises:

introducing a furan-2,5-dicarboxylic acid composition, whichfuran-2,5-dicarboxylic acid composition contains an impurity compoundand which impurity compound is 5-formyl-furan-2-carboxylic acid, into acathode compartment of an electrochemical cell; and

electrochemically reducing the impurity compound in the cathodecompartment.

By electrochemically reducing the impurity compound, i.e. the5-formyl-furan-2-carboxylic acid, the process is suitably producing oneor more impurity reduction products, i.e. one or more of reductionproducts of 5-formyl-furan-2-carboxylic acid.

The process according to the invention conveniently allows one toselectively reduce carbonyl groups, such as the aldehyde group in5-formyl-furan-2-carboxylic acid.

Conveniently such carbonyl groups can be selectively reduced, whilstcarboxyl groups, such as the carboxyl groups of thefuran-2,5-dicarboxylic acid, are essentially maintained intact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a first process according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The furan-2,5-dicarboxylic acid composition used in the processaccording to the invention can conveniently be a composition containingthe furan-2,5-dicarboxylic acid, 5-formyl-furan-2-carboxylic acid, andoptionally one or more additional impurity compounds. By an impuritycompound is herein understood an organic compound other than thefuran-2,5-dicarboxylic acid. As explained in more detail below, suchimpurity compounds may include colored compounds, and as a result oftheir presence the furan-2,5-dicarboxylic acid composition may have adistinguished color.

Preferably the one or more additional impurity compounds contain orconsist of an organic compound comprising a carbonyl group. The processaccording to the invention is especially advantageous where thefuran-2,5-dicarboxylic acid composition contains a, preferably aromatic,organic compound comprising a carbonyl group. The process according tothe invention allows one to selectively reduce such carbonyl group(s).Conveniently the carbonyl group may be selectively reduced, whilst thecarboxyl groups are essentially maintained intact. Such organic compoundcomprising the carbonyl group can suitably be an aldehyde or a ketone.For example, the organic compound comprising the carbonyl group can be adimer or polymer where the monomers are coupled via a carbonyl group.Preferably such an organic compound comprising the carbonyl group is anaromatic compound comprising a carbonyl group, more preferably theorganic compound comprising the carbonyl group is an aromatic compoundcomprising both a carbonyl group and a carboxyl group.

Preferably the furan-2,5-dicarboxylic acid composition contains orconsists of furan-2,5-dicarboxylic acid and 5-formyl-furan-2-carboxylicacid, and optionally a solvent.

The furan-2,5-dicarboxylic acid composition can suitably contain in therange from equal to or more than 1 parts per billion by weight (ppbw),more preferably equal to or more than 1 parts per million by weight(ppmw), to equal to or less than 50 wt %, more preferably equal to orless than 10 wt % of impurity compounds, based on the total weight oforganic compounds in the furan-2,5-dicarboxylic acid composition. Moresuitably the furan-2,5-dicarboxylic acid composition can contain in therange from equal to or more than 10 ppmw to equal to or less than 1 wt %of impurity compounds, based on the total weight of organic compounds inthe furan-2,5-dicarboxylic acid composition. The remaining part of theorganic compounds in the furan-2,5-dicarboxylic acid composition ispreferably made up of the furan-2,5-dicarboxylic acid itself.

More suitably, the furan-2,5-dicarboxylic acid composition can containin the range from equal to or more than 1 parts per billion by weight(ppbw), more preferably equal to or more than 1 parts per million(ppmw), to equal to or less than 50 wt %, more preferably equal to orless than 10 wt % of 5-formyl-furan-2-carboxylic acid, based on thetotal weight of organic compounds in the furan-2,5-dicarboxylic acidcomposition. More suitably the furan-2,5-dicarboxylic acid compositioncan contain in the range from equal to or more than 10 ppmw to equal toor less than 1 wt % of 5-formyl-furan-2-carboxylic acid, based on thetotal weight of organic compounds in the furan-2,5-dicarboxylic acidcomposition. The remaining part of the organic compounds in thefuran-2,5-dicarboxylic acid composition is preferably made up of thefuran-2,5-dicarboxylic acid itself.

In addition to the furan-2,5-dicarboxylic acid and impurity compounds,such as for example the 5-formyl-furan-2-carboxylic acid, thefuran-2,5-dicarboxylic acid composition can contain other non-organiccompounds. For example in addition to the dicarboxylic acid and impuritycompounds, such as for example the aldehyde as described above, thefuran-2,5-dicarboxylic acid composition can contain an electrolytesolution as described below.

Aromatic dicarboxylic acids, such as furan-2,5-dicarboxylic acid, areobtainable by or can suitably be obtained, directly or indirectly, byoxidation of the corresponding dialkyl aromatic compounds.

Furan-2,5-dicarboxylic acid can conveniently be produced by means of aprocess comprising oxidation of 5-hydroxymethylfurfural or ethers and/oresters thereof. Suitable processes have been described in U.S. Pat. Nos.8,865,921 and 8,519,167.

The process according to the invention can be especially advantageouswhere the furan-2,5-dicarboxylic acid composition used as a feed has asignificant color. As mentioned above the furan-2,5-dicarboxylic acidcomposition may include one or more colored compounds, and as a resultof their presence the furan-2,5-dicarboxylic acid composition may have adistinguished color. The color level of the furan-2,5-dicarboxylic acidcomposition can be ascertained visually and/or any change of color canconveniently be measured by the so-called b*-value on the Hunter ColorScale as described in Hunter, “The measurement of Appearance”, Chapter8, pages 102-132, as published by John Wiley & Sons, NY, NY (1975) andin Wyszecki et al. “Color Science, Concepts and Methods, QuantitativeData and Formulae”, 2^(nd) Ed., pages 166-168, John Wiley & Sons, NY, NY(1982). The b*-value of terephthalic acid can further be determinedusing, for example a Spectrophotometer as described for example in U.S.Pat. No. 4,626,598 and as incorporated herein by reference.

Without wishing to be bound by any kind of theory, it is believed thatby means of reduction in the cathode compartment of an electrochemicalcell also some colorants can potentially be removed.

Preferably, the furan-2,5-dicarboxylic acid composition in the processaccording to the invention is partly or wholly obtained by meanselectrochemically oxidizing a feedstock containing afuran-2,5-dicarboxylic acid precursor.

In a preferred embodiment, the process according to the invention is aprocess for producing and treating a furan-2,5-dicarboxylic acidcomposition, wherein the process comprises:

-   -   introducing a feedstock containing a furan-2,5-dicarboxylic acid        precursor into an anode compartment of an electrochemical cell;    -   electrochemically oxidizing the furan-2,5-dicarboxylic acid        precursor in the anode compartment, thereby producing a        furan-2,5-dicarboxylic acid composition containing an impurity        compound;    -   introducing the furan-2,5-dicarboxylic acid composition        containing the impurity compound, into a cathode compartment of        an electrochemical cell; and    -   electrochemically reducing the impurity compound in the cathode        compartment,        As indicated above, the impurity compound is suitably        5-formyl-furan-2-carboxylic acid. By electrochemically reducing        the impurity compound, i.e. the 5-formyl-furan-2-carboxylic        acid, the process is suitably producing one or more impurity        reduction products, i.e. one or more of reduction products of        5-formyl-furan-2-carboxylic acid.

More preferably the feedstock is a feedstock containing afuran-2,5-dicarboxylic acid and a furan-2,5-dicarboxylic acid precursor.Preferably such a feedstock containing a furan-2,5-dicarboxylic acid anda furan-2,5-dicarboxylic acid precursor is the product of an oxidationprocess as described above. Hence, preferably the feedstock is afeedstock containing furan-2,5-dicarboxylic acid and afuran-2,5-dicarboxylic acid precursor, which is preferably obtained orobtainable by oxidation of a corresponding dialkyl aromatic compound.

Preferably the furan-2,5-dicarboxylic acid precursor is5-hydroxymethylfurfural, an ether and/or an ester thereof, or a mixtureof such.

In such a process it may be advantageous to change the pH in betweensteps. That is, it may be advantageous to carry out the oxidation of thefuran-2,5-dicarboxylic acid precursor at a pH of equal to or more than10, whilst carrying out the reduction of the one or more impuritycompounds, such as 5-formyl-furan-2-carboxylic acid, at a pH of equal toor less than 4.

Without wishing to be bound by any kind of theory it is believed thatthe oxidation of a corresponding dialkyl aromatic compound as describedin the prior art results in a crude furan-2,5-dicarboxylic acidcomposition, which crude furan-2,5-dicarboxylic acid compositioncontains the intended furan-2,5-dicarboxylic acid and intermediateproducts, such as for example a formyl-substituted furan carboxylic acid(e.g. 5-formyl-furan-2-carboxylic acid) or a hydroxylmethyl-substitutedfuran carboxylic acid (e.g. 5-hydroxymethyl-furan-2-carboxylic acid.Such intermediate products, for example a 5-formyl-furan-2-carboxylicacid or a 5-hydroxymethyl-furan-2-carboxylic acid, can be suitablefuran-2,5-dicarboxylic acid precursors in the feedstock to the abovementioned anode compartment and can suitably be electrochemicallyoxidized into the furan-2,5-dicarboxylic acid. Any intermediateproducts, such as 5-formyl-furan-2-carboxylic acid, that are not fullyoxidized in such anode compartment remain present in the producedfuran-2,5-dicarboxylic acid composition as an impurity compound.

In the process according to the invention, the furan-2,5-dicarboxylicacid composition is introduced into a cathode compartment of anelectrochemical cell. By a cathode compartment of an electrochemicalcell is herein understood a compartment of an electrochemical cellcontaining a cathode, wherein suitably such cathode is a workingelectrode. By a working electrode is herein preferably understood anelectrode in an electrochemical system at which a reaction of interestis occurring. In addition to the cathode the cathode compartmentsuitably contains a cathodic electrolyte solution. By a cathodicelectrolyte solution is herein suitably understood an electrolytesolution present in the cathode compartment, which solution is incontact with the cathode.

By an anode compartment is herein understood a compartment of anelectrochemical cell containing an anode, wherein suitably such anode isa working electrode. In addition to the anode the anode compartmentsuitably contains an anodic electrolyte solution. By an anodicelectrolyte solution is herein suitably understood an electrolytesolution present in the anode compartment, which solution is in contactwith the anode.

In the process according to the invention the cathode compartment and anoptional anode compartment can suitably be part of the same dividedelectrochemical cell or the cathode compartment can suitably be part ofa first, divided or undivided, electrochemical cell and the optionalanode compartment can suitably be part of a second, divided orundivided, electrochemical cell.

If the electrochemical cell is an undivided electrochemical cell, thecathode, as a working electrode, is suitably accompanied by an anode, asa counter electrode. That is, the undivided electrochemical cell willcontain both a cathode as well as an anode in the cathode compartment.

In a preferred embodiment the process according to the invention is aprocess comprising:

introducing a furan-2,5-dicarboxylic acid composition, whichfuran-2,5-dicarboxylic acid composition contains an impurity compound,into an undivided electrochemical cell, which undivided electrochemicalcell contains a cathode and an anode; and

electrochemically reducing the impurity compound at the cathode.

As indicated above, the impurity compound is suitably5-formyl-furan-2-carboxylic acid. By electrochemically reducing theimpurity compound, i.e. the 5-formyl-furan-2-carboxylic acid, theprocess is suitably producing one or more impurity reduction products,i.e. one or more of reduction products of 5-formyl-furan-2-carboxylicacid.

In such a process the undivided electrochemical cell preferably furthercomprises an electrolyte solution having a pH of equal to or less than4, or a pH of equal to or more than 10.

Other preferences are as outlined above.

If the electrochemical cell is a divided electrochemical cell, thecathode, as a working electrode, is suitably located in a cathodecompartment whilst the anode, whether as counter electrode or as workingelectrode, is suitably located in another compartment.

In a preferred embodiment the process according to the invention is aprocess comprising:

-   -   introducing a furan-2,5-dicarboxylic acid composition, which        furan-2,5-dicarboxylic acid composition contains an impurity        compound, into an divided electrochemical cell, which divided        electrochemical cell contains a cathode in a cathode compartment        and an anode in a separate other compartment; and    -   electrochemically reducing the impurity compound at the cathode.        As indicated above, the impurity compound is suitably        5-formyl-furan-2-carboxylic acid. By electrochemically reducing        the impurity compound, i.e. the 5-formyl-furan-2-carboxylic        acid, the process is suitably producing one or more impurity        reduction products, i.e. one or more of reduction products of        5-formyl-furan-2-carboxylic acid.

In such a process the divided electrochemical cell preferably furthercomprises an electrolyte solution in the cathode compartment having a pHof equal to or less than 4 or equal to or more than 10, more preferablyequal to or more than 13.

Preferably any electrochemical cell in the process according to theinvention is a divided electrochemical cell, which dividedelectrochemical cell includes at least a cathode compartment and ananode compartment, wherein the cathode compartment contains a cathodeand a cathodic electrolyte solution and the anode compartment containsan anode and an anodic electrolyte solution. The anode and cathode ofsuch a divided electrochemical cell are suitably connected to a powersupply, capable of applying a potential over the anode and cathode. Theelectrochemical cell may or may not further be provided with a referenceelectrode. If present, the reference electrode may be a standardhydrogen electrode. Such a standard hydrogen electrode may provide anindication for the potential to cause the reduction reaction.Preferably, however, the electrochemical cell is operated in the absenceof a reference electrode.

The cathode compartment and the anode compartment in such a dividedelectrochemical cell are conveniently separated from each other, forexample by a semi-porous membrane, made from e.g. sintered glass, porousporcelain, polytetrafluoro ethylene (PTFE or Teflon®) or polyolefin suchas polyethylene or polypropylene. The electrolyte solution in thecathode compartment, i.e. the cathodic electrolyte solution, and theelectrolyte solution in the anode compartment, i.e. the anodicelectrolyte solution, can be the same or different.

The cathode and the anode may each be made of a variety of materials.The material of the cathode and the anode may for example eachindependently be chosen from the group consisting of gold, silver,nickel, palladium, platinum, chromium, ruthenium, rhodium, osmium,iridium, indium, bismuth, copper, tin, iron, lead and compounds oralloys thereof and/or hydroxydes and/or oxides thereof. Also carbon canalso be used as the material of either or both the cathode and/or theanode, for example the cathode and/or the anode may contain or comprisegraphite.

The cathode and/or anode, herein also referred to as electrodes, maysuitably comprise noble metal with the oxide and/or hydroxide thereof.Such an electrode may be similar to the one used in the article byGrabowski et al. (Electrochimica Acta, 36 (1991) 1995). It has beenfound that the use of nickel or copper as material for the cathodeand/or anode is advantageous.

The cathode and/or anode material can be present as a rod, plate, mesh,foam, cloth, or in the form of small particles deposited on a carrier,such as a carbon carrier. Preferably the cathode contains or consists ofcopper, nickel, carbon, indium or bismuth, most preferably the cathodecontains or consists of copper. Most preferably the cathode contains orconsists of a copper or nickel mesh or a carbon cloth.

The electrolyte solution(s) can vary widely and can be any electrolytesolution known by the person skilled in the art to be suitable for anelectrochemical reaction as set out in the currently claimed process.Preferably the electrolyte solution is an aqueous electrolyte solution.The electrolyte solution may, however, advantageously also contain orconsist of acetic acid or methanol. Acetic acid and methanol are oftenused as solvent in a process comprising the oxidation of thecorresponding dialkyl aromatic compounds, directly or indirectlyresulting in the furan-2,5-dicarboxylic acid composition as explainedherein above. The process according to the present inventionadvantageously allows one to avoid any crystallization orrecrystallization steps and advantageously allows one to use thefuran-2,5-dicarboxylic acid containing product of a process wherein adialkyl aromatic compound is oxidized in the presence of an acetic acidor methanol solvent directly as feedstock in a process of the currentinvention.

The cathodic electrolyte solution in the cathode compartment containingthe cathode preferably has a pH of equal to or below 4.0 or equal to orabove 10.0. If the electrolyte solution is an aqueous electrolytesolution having a pH of equal to or more than 10.0, such electrolytesolution may also be referred to herein as an alkaline electrolytesolution. The alkalinity can facilitate the dissolution of thefuran-2,5-dicarboxylic acid. Suitably, such an alkaline electrolytesolution can comprise an alkaline compound selected from an alkali metalhydroxide, alkali metal carbonate, alkali metal bicarbonate, ammonia,ammonium carbonate, ammonium bicarbonate, a trialkylamine andcombinations thereof. The use of weak acids and bases, such as carbonateand bicarbonate, has the advantage that they provide a buffering effect.The trialkylamine can suitably contain alkyl groups with 1 to 4 carbonatoms, preferably 1 or 2 carbon atoms. Suitable amines include trimethylamine and triethyl amine. The aqueous electrolyte may comprise such anamount of an alkaline compound that the aqueous electrolyte, in spite ofthe presence of the furan-2,5-dicarboxylic acid, is still alkaline. ThepH of the aqueous electrolyte is then suitably in the range of 10.0 to14.0.

If the electrolyte solution is an aqueous electrolyte solution having apH of equal to or below 4.0, such electrolyte solution may also bereferred to herein as an acidic electrolyte solution. The electrolytesolution may suitably be formed by the combination of water andfuran-2,5-dicarboxylic acid composition, and the ions are provided bythe carboxyl function in the furan-2,5-dicarboxylic acid and optionallyone or more impurity compounds, when such impurity compound(s) alsocomprises a carboxyl group, such as 5-formyl-furan-2-carboxylic acid.Suitably the cathodic electrolyte solution may be an acetic aqueouselectrolyte solution, suitably having a PH value of 0.5 to 4.0.

As already indicated above, the electrolyte solution does not need toconsist of water and ions only. The electrolyte may conveniently alsocomprise one or more organic diluents. Suitable diluents includewater-miscible organic compounds, such as alcohols, acids, aldehydes,ketones or sulfoxides. Examples of suitable diluents include methanol,ethanol, n-propanol, i-propanol, n-butanol, i-butanol and t-butanol,formaldehyde, acetone, acetic acid and dimethylsulfoxide. Theelectrolyte solution may further conveniently contain electrolytesderived from compounds such as sulfuric acid, perchloric acid and salts.A wide variety of electrolytes can be used, including for example sodium(Na+), chloride (Cl—), bromide (Br—), potassium (K+), magnesium (Mg++),zinc (Zn++), calcium (Ca++), phosphate (HPO4-) and bicarbonate (HCO₃—).

The electrolyte solution(s) preferably contain water at least in anamount of 5% wt, based on the total weight of the electrolyte solution,more preferably at least 50% wt, and most preferably at least 90% wt,based on the total weight of the electrolyte solution. The temperatureapplied during the electrochemical reduction and/or electrochemicaloxidation may vary widely. Suitably a temperature is applied wherefuran-2,5-dicarboxylic acid is sufficiently soluble in water. Preferablythe temperature applied during the electrochemical reduction and/orelectrochemical oxidation ranges from equal to or more than 0° C. toequal to or less than 250° C., more preferably from equal to or morethan 135° C. to equal to or less than 200° C.

In the process according to the invention, an electrical potential isapplied to the anode respectively the cathode. If the electrochemicalcell in the process according to the invention is a dividedelectrochemical cell, and the above-mentioned cathode compartment andthe above-mentioned anode compartment are present in the same dividedelectrochemical cell, the electrical potential is suitably appliedbetween such anode and cathode of such divided electrochemical cell. Ifthe above-mentioned cathode compartment is present in a firstelectrochemical cell and the above-mentioned anode compartment ispresent in a second, separate electrochemical cell, the electrochemicalpotential is suitably applied between the cathode, as working electrode,and a different anode, as counter electrode in the first electrochemicalcell and between the anode, as working electrode, and a differentcathode, as counter electrode, in the second electrochemical cell.

As illustrated in the examples, the process can suitably comprisereducing an organic compound comprising a carbonyl group, such as the5-formyl-furan-2-carboxylic acid, thereby producing one or morereduction products. Without wishing to be bound by any kind of theory itis believed that optionally a product composition can be obtainedcontaining the, suitably colorless, furan-2,5-dicarboxylic acid and oneor more colorless reduction products.

The conditions in the electrochemical cell can vary widely, but oneskilled in the art can easily determine the potential and current in theelectrochemical cell of a sufficient magnitude to produce the chemicalreactions desired. The potential difference between anode and cathode inthe electrochemical cell or electrochemical cells, as applicable, issuitably below 10 V, more preferably below 1.23 V. By applying a voltagebelow 1.23 V the electrolysis of water is avoided. The desired voltagecan be provided by installing a predetermined current or currentdensity. The current may vary within wide limits as determined by theshape, size and other parameters of the electrochemical cell. Preferablythe current density is varied between 0.1 mA/cm² and 10 A/cm², morepreferably from 0.2 mA/cm² to 1 A/cm². The total current is adaptedaccording to the surface of the smallest electrode. The reaction istypically prolonged at these conditions until the desired reactions takeplace.

The use of a divided electrochemical cell advantageously allows one tocarry out one conversion in the cathode compartment and anotherconversion in the anode compartment.

The process according to the invention can conveniently be combined withthe processes as described in WO2016/186504 and/or WO2016/186505. Theprocesses as described in WO2016/186504 and/or WO2016/186505 canadvantageously be carried out in the anode compartment of a dividedelectrochemical cell, or a second, separate electrochemical cell, whilstcarrying out the process according to the current invention in thecathode compartment of the divided electrochemical cell, respectively afirst electrochemical cell. As explained above, this conveniently allowsone to prepare a, preferably purified, furan-2,5-dicarboxylic acidcomposition in the anode compartment of the divided electrochemicalcell, whereafter the furan-2,5-dicarboxylic acid composition prepared insuch anode compartment can advantageously to be introduced in thecathode compartment of another or the same divided electrochemical cellto further purify, and optionally decolorize, suchfuran-2,5-dicarboxylic acid composition.

Preferably, the furan-2,5-dicarboxylic acid composition that isintroduced in the cathode compartment of a divided electrochemical cell,contains a dicarboxylic acid which has at least partly been obtained byoxidizing a feedstock containing one or more furan-2,5-dicarboxylic acidprecursors in the anode compartment of such same divided electrochemicalcell. Preferably such feedstock containing one or morefuran-2,5-dicarboxylic acid precursors includes the correspondingalkanol, aldehyde or any mixture thereof of the furan-2,5-dicarboxylicacid.

The present invention therefore also provides a process comprisingtreating a feedstock comprising a furan-2,5-dicarboxylic acid and one ormore furan-2,5-dicarboxylic acid precursors in a divided electrochemicalcell, which divided electrochemical cell comprises a cathode compartmentcontaining a cathode and a cathodic electrolyte solution and an anodecompartment containing an anode and an anodic electrolyte solution,which process comprises:

i) introducing the feedstock into the anode compartment of the dividedelectrochemical cell; and electrochemically oxidizing thefuran-2,5-dicarboxylic acid precursor at the anode to thereby produce afuran-2,5-dicarboxylic acid composition, which furan-2,5-dicarboxylicacid composition contains one or more impurity compounds; andii) introducing the furan-2,5-dicarboxylic acid composition into thecathode compartment of the divided electrochemical cell, andelectrochemically reducing at least part of the one or more impuritycompounds.

As explained above, the one or more impurity compounds may includecolored compounds, and as a result of their presence thefuran-2,5-dicarboxylic acid composition may have a distinguished color.Preferences for the impurity compounds and furan-2,5-dicarboxylic acidcomposition are as described above. The feedstock used in the processcomprising steps i) and ii) above preferably comprises afuran-2,5-dicarboxylic acid obtained by oxidation of a dialkyl aromaticcompound as described herein before. Preferences for the anode materialand cathode material are also as described above. The pH of the anodicelectrolyte solution, present in the anode compartment, may vary widelybut preferably the anodic electrolyte solution has a pH of equal to orless than 4.0. Preferences for the anodic electrolyte solution inrespect of the electrolyte are as described above for the cathodicelectrolyte solution. The oxidation in step i) and the reduction in stepii) are suitably achieved by applying an electrical potential betweenthe anode and the cathode of the electrochemical cell. Preferences forthe potential and current are as described above.

The process according to the invention and the process comprising stepsi) and ii) described above can suitably be carried out batch-wise, semibatch-wise or continuously. When carried out continuously the reductionin the cathode compartment of the divided electrochemical cell and theoxidation in the anode compartment of the divided electrochemical cellare conveniently carried out simultaneously.

The impurity reduction products, such as the one or more reductionproducts of 5-formyl-furan-2-carboxylic acid, mentioned for the aboveprocesses may suitably comprise one or more organic compounds comprisinga hydroxyl group. More suitably the impurity reduction productsmentioned for the above processes may include one or more alcohols, suchas for example 5-hydroxylmethyl-furan-2-carboxylic acid (HMFCA), whichis a reduction product of 5-formyl-furan-2-carboxylic acid.

Conveniently an electrochemically reduced product composition isobtained, comprising furan-2,5-dicarboxylic acid and such5-hydroxylmethyl-furan-2-carboxylic acid. More preferably such anelectrochemically reduced product composition is containingfuran-2,5-dicarboxylic acid and in the range from equal to or more 1ppbw, more preferably equal to or more than 10 ppmw to equal to or lessthan 1 wt % of 5-hydroxylmethyl-furan-2-carboxylic acid, based on thetotal weight of organic compounds in the electrochemically reducedproduct composition. The remainder of the organic compounds preferablyconsist predominantly (i.e. more than 60 wt %) of furan-2,5-dicarboxylicacid.

Advantageously the process according to the invention comprises afurther step of separating one or more impurity reduction products, suchas the one or more reduction products of the 5-formyl-furan-2-carboxylicacid produced by electrochemically reducing such5-formyl-furan-2-carboxylic acid, from the furan-2,5-dicarboxylic acid.Such further step may comprise acidizing an electrolyte solutioncomprising the furan-2,5-dicarboxylic acid and the one or more reductionproducts, thereby allowing the furan-2,5-dicarboxylic acid andoptionally one or more reduced products, to precipitate to form aprecipitated furan-2,5-dicarboxylic acid composition.

Such a composition could optionally comprise an essentially colorlessfuran-2,5-dicarboxylic acid and essentially colorless derivatives of5-formyl-furan-2-carboxylic acid.

FIG. 1 illustrates a non-limiting example of the processes according tothe invention, wherein a feedstock comprising a furan-2,5-dicarboxylicacid and one or more furan-2,5-dicarboxylic acid precursors (101) istreated in a divided electrochemical cell (103), which dividedelectrochemical cell (103) comprises a cathode compartment (105)containing a copper mesh cathode (107) and a cathodic electrolytesolution (109), present on both sides of the copper mesh cathode (107),and an anode compartment (111) containing a copper mesh anode (113) andan anodic electrolyte solution (115), present on both sides of thecopper mesh anode (113). The cathode (107) and anode (113) are connectedto a power supply (117), which power supply provides a suitablepotential over the anode and cathode to achieve the oxidation at theanode and the reduction at the cathode. The cathode compartment (105)and anode compartment (111) are separated from each other by means of asemi-porous membrane made from sintered glass (119). In the exemplaryprocess of FIG. 1, the feedstock (101) is introduced into the anodecompartment (111) via inlet (121). In the anode compartment (111) thefeedstock (101) is contacted with the anode (113) to allow thefuran-2,5-dicarboxylic acid precursors to be oxidized intofuran-2,5-dicarboxylic acid such as to produce a furan-2,5-dicarboxylicacid composition containing 5-formyl-furan-2-carboxylic acid. Theproduced furan-2,5-dicarboxylic acid composition (123) is withdrawn fromanode compartment (111) via outlet (125) and introduced via inlet (127)into the cathode compartment (105), where the furan-2,5-dicarboxylicacid composition (123) is contacted with the cathode (107) and at leastpart of the 5-formyl-furan-2-carboxylic acid is electrochemicallyreduced to produce one or more impurity reduction products, such as5-hydroxymethyl-furan-2-carboxylic acid. The furan-2,5-dicarboxylic acidand one or more impurity reduction products, such as5-hydroxymethyl-furan-2-carboxylic acid, (131) are withdrawn fromcathode compartment (105) via outlet (129) and forwarded to acrystallization unit to crystallize out a purified colorlessfuran-2,5-dicarboxylic acid composition (not shown).

EXAMPLES

The invention is further illustrated by the following non-limitingexamples

Example 1 and Comparative Example 1A: Electrochemical Purification of aCrude furan-2,5-dicarboxylic acid Composition in a 0.5M sodium hydroxideSolution

50 Milligrams of crude furan-2,5-dicarboxylic acid, 1 milliliter of a0.5 molair aqueous sodium hydroxide solution (0.5 M NaOH) and a magneticstirring bar were added to each of the two plastic tubes (Teflon®). Thetubes were each closed with a septum. In Example 1, the solution at thebottom of the tube was placed into contact with a nickel foam anode (asthe working electrode) and a carbon rod cathode (as the counterelectrode). In Comparative Example 1A no electrodes were used.

In both Example 1 as well as in Comparative Example 1A the magneticstirring bar was stirred by placing it on a magnetic stirrer.Subsequently both tubes were heated to a temperature inside the reactorsof 140° C. In example 1 a current of 0.005 ampere was applied for 1800seconds. Hereafter both tubes were cooled and a sample was taken fromeach of the tubes. The content of the samples was determined with gaschromatography and the results are illustrated in Table 1 below. Theconcentrations of 5-hydroxylmethyl-furan-2-carboxylic acid,5-formyl-furan-2-carboxylic acid, and furan-2,5-dicarboxylic acid areshown in milligrams per millilitre (mg/ml).

The solution from Comparative Example 1A showed precipitation after afew hours. The solution of Example 1 did not show such precipitation.

Example 2 and Comparative Example 2A: Electrochemical Purification of aCrude furan-2,5-dicarboxylic acid Composition in a 0.5M sodium hydrogencarbonate Solution

50 Milligrams of crude furan-2,5-dicarboxylic acid, 1 milliliter of a0.5 molair aqueous sodium hydrogen carbonate solution (0.5 M NaHCO₃) anda magnetic stirring bar were added to each of the two plastic tubes(Teflon®). The plastic tubes were each closed with a septum. In Example2, the solution at the bottom of the tube was placed into contact with anickel foam anode (as the working electrode) and a carbon rod cathode(as the counter electrode). In Comparative Example 2A no electrodes wereused.

In both Example 2 as well as in Comparative Example 2A the magneticstirring bar was stirred by placing it on a magnetic stirrer.Subsequently both tubes were heated to a temperature inside the reactorsof 140° C. In Example 2 a current of 0.005 ampere was applied for 1800seconds. Hereafter both tubes were cooled and a sample was taken fromeach of the tubes. The content of the samples was determined with gaschromatography and the results are illustrated in Table 1 below. Theconcentrations of 5-hydroxylmethyl-furan-2-carboxylic acid,5-formyl-furan-2-carboxylic acid, and furan-2,5-dicarboxylic acid areshown in milligrams per millilitre (mg/ml).

TABLE 1 Example Conditions HMFCA FFCA FDCA** 1A * no electrodes, 140°C., 0.00 0.06 8.32 0.5M NaOH 1 Ni anode, C cathode, 0.04 0.01 9.12 140°C., 0.5M NaOH 2A * no electrodes, 140° C., 0.00 0.05 8.08 0.5M NaHCO3 2Ni anode, C cathode, 0.03 0.02 8.54 140° C., 0.5M NaOH * comparative**the experiments were calibrated to measure the HMFCA and FFCAconcentrations rather than the larger FDCA concentrations. HMFCA =5-hydroxylmethyl-furan-2-carboxylic acid FFCA =5-formyl-furan-2-carboxylic acid FDCA = furan-2,5-dicarboxylic acid

1. A process for treating a furan-2,5-dicarboxylic acid composition,which process comprises: introducing a furan-2,5-dicarboxylic acidcomposition, which furan-2,5-dicarboxylic acid composition contains animpurity compound and which impurity compound is5-formyl-furan-2-carboxylic acid, into a cathode compartment of anelectrochemical cell; and electrochemically reducing the impuritycompound in the cathode compartment.
 2. The process according to claim1, wherein the furan-2,5-dicarboxylic acid is obtained or obtainable,directly or indirectly, by oxidation of a corresponding dialkyl aromaticcompound.
 3. The process according to claim 1, wherein the cathodecompartment contains a cathodic electrolyte solution, which cathodicelectrolyte solution has a pH of equal to or below 4.0 or equal to orabove 10.0.
 4. The process according to claim 1, wherein the cathodecompartment contains a cathode containing or consisting of carbon, orcontaining or consisting of a material chosen from the group consistingof gold, silver, nickel, palladium, platinum, chromium, ruthenium,rhodium, osmium, iridium, indium, bismuth, copper, tin, iron, lead andcompounds or alloys thereof and/or hydroxydes and/or oxides thereof. 5.The process according to claim 1, wherein the cathode compartmentcontains a cathode containing or consisting of a copper or nickel meshor a carbon cloth.
 6. The process according to claim 1, comprising afurther step of separating one or more reduction products of the5-formyl-furan-2-carboxylic acid, produced by electrochemically reducing5-formyl-furan-2-carboxylic acid, from the furan-2,5-dicarboxylic acid.7. The process according to claim 6, wherein the further step comprisesacidizing the electrolyte solution comprising the furan-2,5-dicarboxylicacid and the one or more reduction products of the5-formyl-furan-2-carboxylic acid, thereby allowing thefuran-2,5-dicarboxylic acid to precipitate to form a precipitatedfuran-2,5-dicarboxylic acid composition.
 8. A process for producing andtreating a furan-2,5-dicarboxylic acid composition, wherein the processcomprises: introducing a feedstock containing a furan-2,5-dicarboxylicacid precursor into an anode compartment of an electrochemical cell;electrochemically oxidizing the furan-2,5-dicarboxylic acid precursor inthe anode compartment, thereby producing a furan-2,5-dicarboxylic acidcomposition containing an impurity compound; introducing thefuran-2,5-dicarboxylic acid composition containing the impuritycompound, into a cathode compartment of an electrochemical cell; andelectrochemically reducing the impurity compound in the cathodecompartment, thereby producing one or more impurity reduction products.9. The process according to 8, wherein the feedstock is a feedstockcontaining furan-2,5-dicarboxylic acid and a furan-2,5-dicarboxylic acidprecursor.
 10. The process according to claim 8, wherein the cathodecompartment and the anode compartment are part of the same dividedelectrochemical cell.
 11. The process according to claim 8, wherein thecathode compartment is part of a first, divided or undivided,electrochemical cell and the anode compartment is part of a second,divided or undivided, electrochemical cell.
 12. The process according toclaim 8, wherein the furan-2,5-dicarboxylic acid precursor comprises5-hydroxymethylfurfural or an ether or ester thereof.